IEEE 802.3 defines two types of systems for delivering power over an Ethernet link. The original Power over Ethernet (“PoE”) specification (i.e., IEEE 802.3af) governs the supply of up to approximately 15 W power from Power Sourcing Equipment (“PSE”) (e.g., devices that source power over an Ethernet link) to Powered Devices (“PDs”) (e.g., devices that sink power over an Ethernet link). The specification includes a mechanism that allows the PSE to detect that a compliant PD is present so that power is only enabled on the Ethernet link when a compliant PD is connected. Detection and classification of a Type 1 device (e.g., PSE or PD) can be accomplished using single classification state. An enhancement to the original PoE specification was defined to increase the power sourced up to approximately 30 W, as well as improving management through the use of higher level data exchanges. An important aspect of the specification for enhanced PoE (“PoE+”) (i.e., IEEE 802.3at) was the mechanism by which the PSE can definitively detect an enhanced (i.e., Type 2) PD and by which an enhanced PD can definitively understand that it has been detected by an enhanced (i.e., Type 2) PSE. Mutual recognition is necessary before the Type 2 PSE can increase the power available or the Type 2 PD can increase the power drawn. The detection and classification mechanism is designed so that it can be implemented by simple or complex systems, including PSEs that include the data link interface (i.e., end-point PSEs) and PSEs that are inserted in the channel between the two data link partners (i.e., mid-span PSEs). For example, detection and classification of a Type 2 device (e.g., PSE or PD) can be accomplished using two classification states. Additionally, a link layer discovery protocol (“LLDP”) can optionally be used to convey detailed power requirements.
In both the original and enhanced specifications for PoE, the power is delivered over 2 of the 4 twisted pairs in an Ethernet cable. According to these specifications, PSEs are forbidden to deliver power over all 4 twisted pairs and PDs are required to accept power over either set of twisted pairs but are not allowed to require power over all 4 twisted pairs simultaneously.